Recent grid history

Inertia is the kinetic energy of the machines physically spinning on the system — the energy that arrests a frequency fall in the first instant of a loss, before any control can act. Transmission-synchronous is that real inertia computed live from the generation mix, using industry-standard inertia constants (H) — an approximation, and by construction it counts only synchronous plant. GDA inertia model is our estimate of the same real synchronous inertia, settlement-aligned (validated ±5% / ~8.7 GVA·s). Measured (NESO Outturn) is the published lumped scalar — and it includes synthetic. The gap above the model is that synthetic slice: where NESO reports ~100, we resolve it as ~75 real + ~25 synthetic (grey + synthetic ≈ the lumped figure — it decomposes almost cleanly). We refuse to count synthetic as inertia: a fast inverter response isn't present at T+0, so it can't arrest the first swing. Dashed = the ~102 GVA·s FRCR floor.